Such a drive unit usually comprises a motor, a worm shaft and a worm gear. The worm shaft connects the shaft of the motor to the worm gear, and transmits output of the motor to the worm gear. Traditional drive units use a metal worm shaft pressed onto the motor shaft and a metal worm gear. To lower the cost, plastic worms and worm gears have replaced the metal worms and worm gears. However, how to effectively and efficiently transmit the motor torque to a plastic worm shaft is a subject to be resolved. One solution is to make the motor shaft with a D-shaped cross section at a portion where the worm shaft is fixed, and to make the worm shaft with a correspondingly shaped mounting hole. The worm shaft is fixed on to the D-shaped portion and directly driven by the motor shaft. However, this reduces the diameter of the motor shaft resulting in a decrease in the strength of the motor shaft. The motor shaft is vulnerable to bending due to the relatively large output torque of the motor, resulting in poor meshing between the worm shaft and the worm gear, even malfunction of the drive unit.
Hence, there is a desire for an improved drive unit.